Message in a Bottle is a quest in Stranglethorn to kill King Mukla.
<Name>, I put my faith in you and anyone you can muster to help free me. The great Skymane King Mukla is my captor. He commands the beasts of this island, and they keep me here against my will with the aid of this chain. If you can slay him and find the key to this shackle, I shall be indebted to you greatly.
I often hear him to the southeast side of the island, but please be careful: he is not to be trifled with.
I thank my ancestors someone has come to aid me. Do you have the key that will free me?
<name>, you have saved me from a great dishonor. If it was within my power, I would offer you a place among my tribe. But for now, all I can only offer you is this token for the courage and nobility you have shown.
Quest progression Edit
Patch 2.3 changed the quests level, along with Jaguero Island which was brought down to the 40 area. Prior to the patch King Mukla was an elite level 51 Gorilla.